A "gccgoprefix" flag is added and used by the go tool,
to mirror the -fgo-prefix flag for gccgo, whose value
is required to know how to access functions from C.

Trying to export Go methods or unexported Go functions
will not work.

Also fix go test on "main" packages.

Updates #2313.
Fixes #3262.

R=mpimenov, rsc, iant
CC=golang-dev
https://golang.org/cl/5797046